The production of oil from underground reservoirs results in crude oil containing varying amounts of water generally in the form of a water-in-oil emulsion. It is general practice to dehydrate the crude oil by allowing it to stand but oftentimes the dehydration is enhanced by the addition of a demulsifier to break the emulsion facilitating physical separation of the crude oil from the water. Following this dehydration step, the crude oil is transported to the refinery where it may undergo an initial dewatering procedure and/or subjected to the process of desalting, i.e. the removal of salts from hydrocarbon crude oil, sometimes employing the action of an electrocoalescer.
Salts in hydrocarbon crude oil are generally dissolved in small droplets of water or brine dispersed throughout the crude. Sodium chloride is the primary salt followed by calcium chloride, magnesium chloride and the sulfates of these three metals. The total salt content ranges from substantially zero to several hundred pounds per thousand barrels of crude.
These brine droplets are generally prevented from coalescing and settling by a tough, elastic film at the surface of each droplet. This film is stabilized by natural emulsifiers found in the crude, solids, and solid hydrocarbons that concentrate at the droplet surface. A desalting chemical or demulsifier displaces these natural emulsifiers and solids and weakens the film so the droplets of brine can coalesce when they contact each other.
A new oil field will frequently produce crude with negligible water and salt. As production continues, the amount of water produced increases, raising the salt content of the crude. Additional salt contamination often occurs during tanker shipment. An empty tanker takes on sea water as ballast and often uses it to wash the tanks. To minimize pollution, the top, oily layer of ballast water and the washings are segregated in a slop compartment when the ballast water is discharged. Fresh crude is then loaded on top of this slop oil and water. The entire compartment is then offloaded at the refinery.
As earlier inferred, some brine can be removed by settling and water drawoff in the refinery's crude storage tanks. Some demulsifiers are very effective in increasing the rate and amount of settling as well as preventing sludge buildup and in cleaning tanks where sludge has already accumulated. Typically, the demulsifier formulation is injected into the turbulent crude flow as it fills the storage tank at a treat rate of from 10 to 500 ppm. The settled brine is drawn before the crude is charged to the pipestill.
To enhance the effectiveness of electrostatic desalter, desalting chemicals are used in combination with an imposed electric field. Desalting chemicals are usually a blend of surface active materials in hydrocarbon solvents. These materials are preferentially absorbed at the brine droplet surface, displacing the solids and natural emulsifiers. This greatly weakens the film around the droplets. The brine droplets can then coalesce with the wash water (thus diluting the brine) and with other droplets so their size becomes large enough to settle by gravity. Depending on its composition and solvent, the desalting chemical may also dissolve the film.
To overcome solids stabilization of an emulsion, a good demulsifier formulation will cause the oil-wet solids to become water-wet and settle into the water phase where they are removed with the effluent water. A surfactant can also be used alone or in combination with the demulsifier for this purpose. These chemicals work by attaching an oil-loving or solids-loving section of the molecule to an oil-wetted solid. A water-loving section then physically drags the solid into the water phase. These molecules can also agglomerate solids to speed their settling. Without chemical treatment, most oil-wet solids will stay in the oil phase even though their density is higher.
With the rising value of petroleum products, it becomes increasingly important that separator equipment utilized by the petroleum industry extract the maximum possible recovered petroleum products from oil and water emulsions and dispersions. These are main issues that currently affect the profitability and operating integrity related to oil carryover in effluent water streams from dewatering/desalting systems.
There exists a need to continuously monitor the oil carryover in effluent water streams to ensure that maximum profitability can be maintained.